Uniforms are one of the few good things we got from the military. Why do you think they wear them? Because it helps them do the job they have to do. The same applies to aviation. The uniform is one of the tools you use to help do your job better And the size of the aeroplane has little to do with it. Many pilots in small aircraft do difficult and demanding work.
There does not seem to be anything written on the subject, but general industry practise seems to be.
No-one with a CPL or higher wears one bar. That seems to be for cadets who are not on line yet.
Two is for an active line pilot, and three is for a more senior line pilot.
Ansett used to say that four bars indicated a person who has been a captain for more than ten years.
This sounds fairly sensible, and pilots should wear uniforms when dealing with the public. Wear it properly, behave in a responsible manner when wearing it (like always), and NEVER be seen drinking alcohol in uniform.